Reduction of field edge thinning in peripheral devices

ABSTRACT

A dielectric layer (e.g., an interpoly dielectric layer) is deposited over low and high voltage devices of a peripheral memory device. The dielectric behaves as an oxidation and wet oxide etch barrier. The dielectric prevents the devices from being stripped by a wet oxide etch that can result in the exposure of the silicon corners. The exposure of a silicon corner may increase thinning of a gate oxide at the field edge. This causes variability and unreliability in the device. The dielectric is not removed from a device until the device is ready for processing. That is, the dielectric remains on a device until the growing of a gate oxide on that device has begun. This reduces the exposure of the silicon corner. Hedges that result may be removed by exposing a trench in the field oxide at the hedge.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.10/180,415, filed Jun. 24, 2002, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to reducing gate oxide thinning at the field edgeof complimentary metal oxide semiconductor (CMOS) devices and, moreparticularly, to reducing field edge thinning in peripheral devices.

Some CMOS devices are fabricated in such a way that a thin gate oxideand a thick gate oxide are grown on the device. In some processes forfabricating these peripheral CMOS devices, the thin gate oxide devicesare sometimes stripped with a wet oxide etch prior to gate oxidation. Inother processes, the thick gate oxide devices are sometimes strippedwith a wet oxide etch prior to gate oxidation. When a device is strippedwith a wet oxide etch, the silicon corners at the field edges may beexposed. This results in increased gate oxide thinning at the fieldedges (e.g., at the exposed corners). This increased thinning may leadto undesirable variability in the electrical parameters of theperipheral devices.

For example, the increased thinning at the field edges may cause aMOSFET (metal oxide semiconductor field effect transistor) to break downat the corners due to the non-uniformity of the gate oxide. This resultsin what is sometimes called a threshold kink. That is, a MOSFET may turnon closer to the device's field edges before the rest of the deviceturns on. When enough voltage is applied to the rest of the gate, therest of the device may turn on. In essence, this results in two devicesin parallel. This variability and other factors may lead to degradationin functionality, yield, and reliability in these devices.

It would therefore be desirable to be able to provide fabricationprocesses that reduce field edge thinning in peripheral devices.

SUMMARY OF THE INVENTION

It is an object of this invention to provide fabrication processes thatreduce field edge thinning in peripheral devices.

Some peripheral devices (e.g., CMOS) include a low voltage device and ahigh voltage device separated by a field oxide or isolation dielectric.Field oxides or isolation dielectrics may also separate these peripheraldevices from other devices. The invention is directed to the fabricationof these low and high voltage devices.

Low voltage and high voltage devices are typically fabricated insilicon. The field oxide may be a dielectric such as silicon dioxide.The fabrication processes of the invention eliminate a wet oxide etchused in other fabrication processes. Eliminating the wet oxide etchreduces the exposure of the silicon corners of the peripheral devices attheir field edges. Reducing the exposure of the silicon corners resultsin more uniform gate oxides for such devices. More uniform gate oxidesleads to less variable and more reliable devices.

The invention uses a dielectric layer as an oxidation and wet oxide etchbarrier to prevent exposure of the silicon corners of the thick and thingate oxide devices. Thinning of the gate oxides at the field edges istherefore reduced. The dielectric layer may be an interpoly dielectriclayer, which may be, for example, an oxide-nitride-oxide.

The dielectric layer is deposited on both the low voltage and highvoltage devices of a peripheral device. The low voltage device willultimately have a thin gate oxide grown over it, while the high voltagedevice will ultimately have a thick gate oxide grown over it. Generally,the dielectric layer is not removed from a low or high voltage deviceuntil that device is ready to be processed. That is, the dielectriclayer remains on a device until the process of growing a gate oxide onthe device has begun. Masks may be deposited on or placed over thedevice to selectively etch dielectric layers.

A portion of the thick gate oxide may be grown on a high voltage devicebefore the thin gate oxide is grown on a low voltage device. That is,the gate oxide for the high voltage device may be the thick gate oxidegrown over the high voltage device and an additional layer of oxidegrown over the device (e.g., a thin layer of oxide grown over the lowand high voltage devices—the thin layer of oxide grown over the lowvoltage device may be the gate oxide for that device). In other words,the thin gate oxide is grown directly over the thick gate oxide. Thethin and thick gate oxides grown over the high voltage device will bethe gate oxide for that device.

When masks are not lined up exactly with the dielectric layers they areintended to cover, a small portion of the dielectric layer may not beremoved from the device when oxides are grown. These small portions aresometimes referred to as hedges. Hedges may be removed by exposing atrench in the field oxide near the hedge. Field oxides may be locallyexposed by depositing or placing a mask with openings or windows on orover the device. The field oxide is then etched at the hedges which willin turn remove the hedges.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIGS. 1-6 illustrate a fabrication process that may result in gate oxidethinning at field edges;

FIG. 7 illustrates gate oxide thinning at a field edge because ofincreased exposure of a silicon corner;

FIGS. 8-17 illustrate an embodiment of a fabrication process thatreduces gate oxide thinning at field edges in accordance with theinvention;

FIG. 18 illustrates a portion of a dielectric layer unintentionallycovered by a mask;

FIG. 19 illustrates a hedge that may result from a portion of adielectric layer unintentionally covered by a mask;

FIG. 20 illustrates hedge removal by exposing a trench in a field oxidein accordance with the invention;

FIG. 21 is a flow chart of the fabrication process illustrated in FIGS.8-20 and 33; and

FIG. 22 is a flow chart of another embodiment of a fabrication processthat reduces gate oxide thinning at field edges in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to the fabrication of peripheral devices.Peripheral devices may be used to implement storage cells in memorydevices such as, for example, SRAMs, DRAMs, and memory devices used inflash applications.

In the fabrication of some peripheral memory devices, a dielectric layeris deposited over the peripheral devices and the memory devices to whichthe peripheral devices are coupled. The dielectric layer may be aninterpoly dielectric layer. Such an interpoly dielectric layer may be,for example, oxide-nitride-oxide or aluminum-oxide. The dielectric isused as an oxidation and wet-oxide etch barrier. In some peripheraldevice fabrication, the dielectric layer is removed from the entireperipheral device but not the memory device. The dielectric layer isremoved from the peripheral device to allow thin and thick gate oxidesto be grown over the peripheral device. That is, the low and highvoltage devices are exposed before the growth of the thin and thick gateoxides.

In processes such as these, one device may be stripped more times with awet oxide etch prior to gate oxidation than the other device (e.g., alow voltage device compared to a high voltage device). This increasesexposure of the silicon corners at the field edges resulting inincreased gate oxide thinning at the field edges.

FIGS. 1-6 and 8-20 show peripheral devices having sources and drainsperpendicular to face of the page.

FIGS. 1-6 illustrate a process of fabricating complimentary metal oxidesemiconductor (CMOS) field effect transistor (FET) peripheral devices.As illustrated in FIGS. 1-6, a thin gate oxide device (depicted on theleft of the FIGS.) may see more wet oxide strips prior to gate oxidationthan a thick gate oxide device (depicted on the right of the FIGS.).FIGS. 1-6 illustrate a process that results in the exposure of siliconcorners in low voltage devices. Other processes may result in theexposure of silicon corners in high voltage devices.

Thin gate oxide devices and thick gate oxide devices are sometimesreferred to as low voltage and high voltage devices, respectively. Thethin and thick gate oxide devices are respectively referred to as suchbecause of the amount of voltage required to turn the devices on.Hereinafter, “low voltage device” and “high voltage device” are used torefer to thin gate oxide and thick gate oxide devices, respectively.

As mentioned above, a dielectric layer is sometimes deposited over amemory device as well as peripheral devices. FIG. 1 illustrates aperipheral device 10 with a dielectric layer removed from the entiredevice in order to grow thin and thick gate oxides over the low voltagedevice 20 and the high voltage device 30, respectively. The dielectriclayer is not removed from the rest of the memory device (not shown) inorder to protect the rest of the memory device from the processing ofthe peripheral devices. Devices 20 and 30 are preferably silicondevices. A field oxide 15 separates devices 20 and 30 from each other aswell as from other devices. Field oxide 15 may be any suitable oxidesuch as silicon dioxide.

As shown in FIG. 2, an oxide layer 40 is grown over peripheral device 10after the dielectric layer is removed. Oxide layer 40 will be part ofthe gate oxide for high voltage device 30. The remainder of the gateoxide for high voltage device 30 is grown when the gate oxide for lowvoltage device 20 is grown. After oxide layer 40 is grown, a mask 50 islaid over device 10 such that low voltage device 20 is not covered, asshown in FIG. 3. Mask 50 may be any suitable mask such as an implantmask. The portion of oxide layer 40 that is not covered by mask 50 isthen stripped by a wet oxide etch, as shown in FIG. 4. This wet oxideetch increases the exposure of the silicon corners 60 at the fieldedges. Mask 50 is then removed, as shown in FIG. 5. Another oxide layeris then grown over devices 20 and 30, as shown in FIG. 6. Oxide layer 70is grown directly over oxide layer 40. The oxide layers grown over highvoltage device 30, (i.e., oxide layers 40 and 70) are the gate oxide fordevice 30. That is, the thickness of the gate oxide for device 30 is thesum of the thicknesses of oxide layer 40 and oxide layer 70. Oxide layer70 is the gate oxide for low voltage device 20. In this example, oxidelayer 70 is thinner than oxide layer 40. (None of the FIGS. are drawn toscale.)

FIG. 6 illustrates the increased thinning of oxide layer 70 at the fieldedges because of the increased exposure of the silicon corners of lowvoltage device 20. FIG. 7 is a more detailed illustration of FIG. 6. Asshown, oxide layer 70 is thinner at silicon corner 60 than at the restof the device.

Although the processes of the invention relate primarily to thefabrication of CMOS peripheral devices, these processes may be used toreduce or prohibit gate oxide thinning at field edges in other devicessuch as, for example, capacitors, other types of field effecttransistors, and other related MOS devices.

The fabrication processes in accordance with the invention reduce theincreased gate oxide thinning at the field edge by, among other things,using a dielectric layer as an oxidation and wet oxide etch barrier forthe low and high voltage devices. FIGS. 8-17 illustrate an exemplaryprocess of reducing the exposure of silicon corners and the increasedgate oxide thinning at field edges. In FIGS. 8-17, a low voltage device90 is shown on the left, and a high voltage device 100 is shown on theright.

FIG. 8 illustrates a peripheral device 80 before gate oxides are grownon low voltage device 90 and high voltage device 100. As illustrated inFIG. 9, a dielectric layer 110 is deposited over devices 90 and 100.Dielectric layer 110 may be any suitable dielectric layer that behavesas an oxidation and wet oxide etch barrier. Such a dielectric layer maybe an interpoly dielectric layer of, for example, oxide-nitride-oxide oraluminum-oxide. Dielectric layer 110 preferably has a thickness suitableto suppress oxidation and behave as a wet oxide etch barrier. For adielectric layer 110 of oxide-nitride-oxide, such a thickness may be,for example, about 150 angstroms.

After dielectric layer 110 is deposited, a mask 120 is deposited on orplaced over peripheral device 80 such that the portion of dielectriclayer 110 covering high voltage device 100 can be removed, as shown inFIG. 10. As shown in FIG. 11, the exposed portion of dielectric layer110 is removed. This removal may be accomplished by a dry etch, a wetetch, or a combination of a dry and wet etch. Generally, those portionsof a dielectric layer that are covered by a mask (e.g., mask 120) arepreferably not removed. That is, when a mask is deposited on or placedover portions of a dielectric layer, those portions are preferably notremoved by, for example, a dry or wet etch.

As illustrated in FIGS. 10 and 11, mask 120 may be an implant mask.Alternatively, mask 120 may be any other suitable type of mask such thatportions of dielectric layer 110 may be selectively removed and oxidelayers (e.g., oxide layer 130) selectively grown. Mask 120 may be of anysuitable material such as a photoresist.

As illustrated in FIG. 12, mask 120 is removed after the exposed portionof dielectric layer 110 is removed. Oxide layer 130 is then grown overhigh voltage device 100, as shown in FIG. 13. The portion of dielectriclayer 110 covering low voltage device 90 prevents the growth of anyoxide over low voltage device 90.

Oxide layer 130 is preferably a portion of the total gate oxide for highvoltage device 100. That is, the fabrication process may subsequentlygrow more oxide on top of oxide layer 130. Oxide layer 130 inconjunction with additional oxide may serve as the gate oxide for device100.

After oxide layer 130 is grown over high voltage device 100, a mask 140is deposited on or placed over oxide layer 130, as shown in FIG. 14. Theportion of dielectric layer 110 protecting low voltage device 90 is thenremoved, as shown in FIG. 15.

As illustrated in FIGS. 14 and 15, mask 140 may be an implant mask. Mask140 alternatively may be any other suitable type of mask that permitsportions of dielectric layer 110 to be selectively removed. Mask 140 maybe of any suitable material such as a photoresist.

FIG. 16 shows mask 140 removed. Oxide layer 150 is then grown over lowvoltage device 90 and high voltage device 100. As illustrated in FIG.17, oxide layer 150 may be grown directly over oxide layer 130. Whenoxide layer 150 is grown over oxide layer 130, less oxide will be grownover oxide layer 130 than over silicon. This occurs because the growthof oxide on silicon is greater than the growth of oxide on oxide. Forexample, if about 30 angstroms are grown on low voltage device 90, onlyabout 20 angstroms of oxide may be grown on high voltage device 100.

During the fabrication of peripheral devices, masks may not line upexactly with the oxide layers or dielectric layers that they areintended to cover. At times, a mask may unintentionally cover a smallportion of a dielectric layer. This is illustrated in FIG. 18. As shown,mask 310 covers a portion of dielectric layer 320. Portion 330 will notbe removed when the rest of dielectric layer 320 is removed, and asshown in FIG. 19, portion 330 may remain on the peripheral device andhave oxide 332 grown over it.

Portions such as portion 330 are sometimes referred to as hedges. Hedgesare often undesirable because they may lift off in subsequentfabrication processing, unpredictably disrupting the grown oxide layer.Hedges may be controllably removed by exposing trenches in the fieldoxide under the hedges. To expose a trench in the field oxide, a maskmay be deposited on or placed over a device. The mask may have openingsthat line up with desired trench locations (i.e., where the hedges arelocated). The field oxide is exposed at those openings, and the mask isthen removed.

As illustrated in FIG. 20, a trench 340 was exposed in field oxide 350causing portion 330 (i.e., a hedge) to be removed. Hedges may bealternatively removed using any suitable method or technique. (FIGS.18-20 are not drawn to scale.)

The fabrication process illustrated in FIGS. 8-17 and 20 is depicted inthe flow chart of FIG. 21. At step 2102, a dielectric layer is depositedover the low voltage and high voltage devices. At step 2104, a firstmask is then deposited on or placed over the low voltage device. Then,at step 2106, the dielectric layer is removed from the high voltagedevice. The first mask is then removed from the low voltage device and afirst oxide layer is grown on the high voltage device (steps 2108 and2110).

Subsequently, a second mask may be deposited on or placed over the highvoltage device (step 2112). The dielectric layer is then removed fromthe low voltage device at step 2114. Then, the second mask is removedfrom the high voltage device and a second oxide layer is grown on thelow voltage device (steps 2116 and 2118). Any hedges that result fromthis process may be removed by exposing a trench in the field oxide atstep 2120.

FIG. 22 depicts another embodiment of a process that reduces gate oxidethinning at field edges in accordance with the invention. At step 2202,a dielectric layer is deposited over a peripheral device. The peripheraldevice includes a first device and a second device (e.g., a low voltagedevice and a high voltage device, MOS devices in parallel, and variousother CMOS devices; capacitors, field effect transistors). At step 2204,a first mask is deposited on or placed over the first device of theperipheral device to protect the dielectric from being removed from thefirst device. At step 2206, the dielectric not protected by the mask isremoved. This exposes the second device of the peripheral device. Thatis, the second device is no longer protected by the dielectric.

The first mask is then removed from the first device and a first oxidelayer is grown on the second device (steps 2208 and 2210). A second maskis then deposited on or placed over the second device (step 2212). Thedielectric layer is then removed from the first device at step 2214.Then, the second mask is removed from the second device and a secondoxide layer is grown on the first device (steps 2216 and 2218). Anyhedges that result from this fabrication process may be removed byexposing a trench in the field oxide at step 2220.

Thus it is seen that fabrication processes for reducing field edgethinning in peripheral devices are provided. One skilled in the art willappreciate that the invention can be practiced by other than thedescribed embodiments, which are presented for purposes of illustrationand not of limitation, and the invention is limited only by the claimswhich follow.

1. A method of fabricating metal oxide semiconductor devices inparallel, the metal oxide semiconductor devices comprising a low voltagesemiconductor device and a high voltage semiconductor device, whereinthe low voltage and high voltage devices are separated by an insulationbarrier, the method comprising: depositing on the semiconductor devicesa dielectric comprising at least one dielectric layer that is inphysical contact with the semiconductor devices; overlaying the lowvoltage semiconductor device with a first mask; preventing theinsulation barrier adjacent the high voltage device from being removed;removing the dielectric from the high voltage semiconductor device wherethe first mask is not overlaid; removing the first mask; growing a firstoxide layer on the high voltage semiconductor device; overlaying thehigh voltage semiconductor device with a second mask; preventing theinsulation barrier adjacent the low voltage device from being removed;removing the dielectric from the low voltage semiconductor device wherethe second mask is not overlaid; removing the second mask; growing asecond oxide layer on the low voltage semiconductor device and the highvoltage semiconductor device; and exposing a trench in the insulationbarrier to remove a hedge.
 2. The method of claim 1 wherein the lowvoltage semiconductor device comprises a field effect transistor.
 3. Themethod of claim 1 wherein the high voltage semiconductor devicecomprises a field effect transistor.
 4. The method of claim 1 whereinthe low voltage semiconductor device comprises a capacitor.
 5. Themethod of claim 1 wherein the high voltage semiconductor devicecomprises a capacitor.
 6. The method of claim 1 wherein the insulationbarrier comprises an isolation dielectric.
 7. The method of claim 1wherein the dielectric comprises aluminum-oxide.
 8. The method of claim1 wherein the dielectric comprises oxide-nitride-oxide.
 9. The method ofclaim 1 wherein the dielectric is about 150 angstroms thick.
 10. Themethod of claim 1 wherein the insulation barrier comprises silicondioxide.
 11. A method of fabricating a peripheral memory devicecomprising a low voltage device and a high voltage device, wherein thelow voltage and high voltage devices are separated by an insulationbarrier comprising silicon dioxide, the method comprising: depositing onthe peripheral device a dielectric comprising at least one dielectriclayer that is in physical contact with the peripheral device; overlayingthe low voltage device with a first mask; preventing the insulationbarrier adjacent the high voltage device from being removed; removingthe dielectric from areas not overlaid with the first mask; removing thefirst mask; growing a first oxide layer on the high voltage device;overlaying the high voltage device with a second mask; preventing theinsulation barrier adjacent the low voltage device from being removed;removing the dielectric from areas not overlaid with the second mask;removing the second mask; growing a second oxide layer on the low andhigh voltage devices; and exposing a trench in the silicon dioxide toremove a hedge.
 12. The method of claim 6 wherein the dielectriccomprises aluminum-oxide.
 13. The method of claim 11 wherein thedielectric comprises oxide-nitride-oxide.
 14. The method of claim 11wherein the dielectric is about 150 angstroms thick.
 15. The method ofclaim 11 wherein the insulation barrier further comprises an isolationdielectric.